The Book of Prayers
حَدَّثَنَا أَحْمَدُ بْنُ حَنْبَلٍ، حَدَّثَنَا مُعْتَمِرُ بْنُ سُلَيْمَانَ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يَعْرِضُ رَاحِلَتَهُ وَهُوَ يُصَلِّي إِلَيْهَا .Ibn 'Umar said:The Apostle of Allah (ﷺ) used to place his camel (towards the Ka'ba) and said prayer in its direction
From Sahih Muslim, whose reports the scholars accept as authentic.